Full phase shifting mask in damascene process

ABSTRACT

A full phase shifting mask (FPSM) can be advantageously used in a damascene process for hard-to-etch metal layers. Because the FPSM can be used with a positive photoresist, features on an original layout can be replaced with shifters on a FPSM layout. Adjacent shifters should be of opposite phase, e.g. 0 and 180 degrees. In one embodiment, a dark field trim mask can be used with the FPSM. The trim mask can include cuts that correspond to cuts on the FPSM. Cuts on the FPSM can be made to resolve phase conflicts between proximate shifters. In one case, exposing two proximate shifters on the FPSM and a corresponding cut on the trim mask can form a feature in the metal layer. The FPSM and/or the trim mask can include proximity corrections to further improve printing resolution.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/035,788 entitled “Full Phase Shifting Mask In Damascene Process”filed Jan. 13, 2005 which is a continuation of U.S. patent applicationSer. No. 10/295,575 entitled “Full Phase Shifting Mask In DamasceneProcess” filed Nov. 14, 2002, which is related to and claims the benefitof priority of the provisional application 60/363,674 filed 11 Mar.2002, entitled “Full Phase Mask in Damascene Process”, having inventorChristophe Pierrat, and assigned to the assignee of the presentapplication.

This application is related to and claims the benefit of priority of thenon-provisional application Ser. No. 09/669,368 filed 26 Sep. 2000,entitled “Phase Shift Masking for Intersecting Lines/”, having inventorChristophe Pierrat, and assigned to the assignee of the presentapplication.

This application is related to and claims the benefit of priority of thenon-provisional application Ser. No. 09/932,239 filed 17 Aug. 2001,entitled “Phase Conflict Resolution for Photolithographic Masks”, havinginventors Christophe Pierrat, et. al., and assigned to the assignee ofthe present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

A full phase shifting mask for patterning a metal layer in an integratedcircuit is described. In particular, the full phase shifting mask can beused with a damascene process, thereby allowing hard-to-etch materials,such as copper, to be used for the metal layer.

2. Description of the Related Art

A standard binary mask includes a patterned opaque (e.g. chrome) layerformed on a transparent (e.g. quartz) substrate. The pattern can betransferred onto the wafer using optical lithography. Specifically, foreach layer of the circuit design, a radiation (e.g. light) source isshone on the mask (wherein the term mask can also refer herein to areticle) corresponding to that layer. This radiation passes through thetransparent regions of the mask and is blocked by the opaque regions ofthe mask, thereby selectively exposing a photoresist layer on the wafer.

The areas in the photoresist layer exposed to the radiation, i.e.irradiated areas, are either soluble or insoluble in a specific solvent,called a developer. If the irradiated areas are soluble, then thephotoresist is called a positive photoresist. In contrast, if theirradiated areas are insoluble, then the photoresist is called anegative photoresist. After development of the photoresist layer, theunderlying semiconductor layer no longer covered by photoresist can beremoved by an anisotropic etch, thereby transferring the desired patternonto the wafer. This process can be repeated for each layer of theintegrated circuit design on the wafer.

A conventional process for patterning a metal layer comprises depositingthat metal layer on the wafer and then depositing a positive photoresistlayer on the metal layer. The positive photoresist can then be exposedusing a clear field binary mask (wherein the opaque pattern on the maskrepresents features in the layout). At this point, etching can beperformed to generate the desired pattern in the metal layer.

This process works well for metal patterns having critical dimensionsgreater than 0.13 microns. However, to enhance device performance atsmaller critical dimensions, the semiconductor industry is moving fromaluminum to copper. Unfortunately, copper is very difficult to etch.Therefore, a conventional metal process as described above cannot beused for a copper layer.

However, a damascene process can be used to form a copper pattern. Thedamascene process can include forming an oxide layer on the wafer andthen depositing a negative photoresist layer on the oxide layer. Thenegative photoresist can be exposed using the clear field binary mask.After exposure, the exposed portions of the oxide layer can be easilyetched to form the desired pattern. At this point, copper can bedeposited and planarized (e.g. using a CMP operation), thereby formingthe desired pattern in copper.

However, positive photoresists are currently the dominant resists formany applications as they provide better resolution than negativephotoresists. Therefore, a need arises for a technique of patterning ametal layer, particularly a hard-to-etch metal, using a positivephotoresist.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a type of phase shiftingmask (PSM) can be advantageously used in a damascene process. Thedamascene process can include the development of a positive photoresist,thereby ensuring optimal resolution of the metal pattern. Of importance,the inherent qualities of a PSM and a positive photoresist facilitatethe conversion of an original layout to a PSM layout.

In one embodiment, a mask set for patterning a metal layer in anintegrated circuit is provided. The mask set can include a full phaseshifting mask (FPSM) and a dark field trim mask. The FPSM includes aplurality of shifters, wherein the shifters define most features in themetal layer. The dark field trim mask can include at least a first cut.This first cut corresponds to a second cut on the FPSM, wherein thesecond cut resolves a phase conflict on the FPSM. In one case, exposingtwo proximate shifters on the FPSM and the first cut on the trim maskcan form a feature in the metal layer.

The FPSM can further include one or more assist shifters, sometimes alsocalled assist bars or scattering bars. Assist shifters, which are verysmall and therefore do not print, nonetheless aid in printingresolution. Assist shifters can be placed on either side of isolatedshifters, placed alongside isolated edges of one or more sets of denselypacked shifters, and/or interspersed with a plurality of intermediatespaced shifters. In one embodiment, the FPSM and/or the trim mask caninclude other proximity corrections. These proximity corrections couldbe provided by either rule-based optical proximity correction (OPC) ormodel-based OPC. Although the term optical proximity correction is usedherein it is used generically to refer to any type of proximitycorrection, e.g. resist, etch, micro-loading, etc.

An exemplary technique of making a phase shifting mask (PSM) is alsoprovided. In this technique, a layout for defining a plurality offeatures in a metal layer can be received. This layout can be converted,if necessary, so that most of the features in the layout are representedby shifters in the PSM (called an FPSM herein). In one embodiment, bothcritical and non-critical features can be represented by shifters in aone-to-one correspondence. If a phase conflict occurs in the convertedlayout, then a feature associated with the phase conflict can be cut,thereby creating two shifters. At this point, one of the two shifterscan be changed to a different phase. The converted layout can then betransferred to the FPSM using a known mask writing process. The FPSM canbe used for patterning a metal layer, such as copper, in a damasceneprocess.

An exemplary technique for patterning the metal layer is also provided.In this technique, an oxide layer can be deposited on a wafer. Then, apositive photoresist layer can be deposited on the oxide layer. At thispoint, the positive photoresist layer can be exposed with a full phaseshifting mask (FPSM) and a trim mask. The FPSM includes a plurality ofshifters, wherein the shifters represent most features in the metallayer. In one embodiment, the trim mask is a dark field trim mask withat least one cut. This cut corresponds to a cut on the FPSM, wherein thecut on the FPSM resolved a phase conflict on the FPSM. At this point,the positive photoresist layer can be developed and the exposed portionsof the oxide layer can be etched, thereby transferring the desiredpattern to the oxide layer. Then, the metal layer can be deposited onthe wafer and planarized to substantially a top surface of the etchedoxide layer. In this manner, the desired pattern has been transferred tothe metal layer without etching of the metal. This damascene process isparticularly useful for hard-to-etch metals, such as copper.

BRIEF DESCRIPTION OF THE FIGURES

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Patent and Trademark Officeupon request and payment of the necessary fee.

FIG. 1A illustrates a full phase shifting mask (FPSM) layout for formingthree lines in a metal layer.

FIG. 1B illustrates a trim layout that corresponds to the FPSM layout ofFIG. 1A. Specifically, the trim layout can eliminate extraneous featurescreated by the FPSM layout.

FIG. 1C illustrates an aerial image after exposing two masksimplementing the FPSM layout of FIG. 1A and the trim layout of FIG. 1B.

FIG. 2A illustrates a FPSM layout including proximity corrections,wherein the modified FPSM layout can form three lines in a metal layer.

FIG. 2B illustrates a trim layout that corresponds to the FPSM layout ofFIG. 2A.

FIG. 2C illustrates a printed image after exposing masks implementingthe FPSM layout of FIG. 2A and the trim layout of FIG. 2B.

FIG. 3A illustrates a FPSM layout including an isolated shifter and twoassist shifters placed on either side of the isolated shifter.

FIG. 3B illustrates a FPSM layout including multiple densely spacedshifters with assist shifters placed at their periphery.

FIG. 3C illustrates a FPSM layout includes intermediate spaced (i.e.between isolated and densely spaced) shifters with interspersed assistshifters.

FIG. 3D illustrates a FPSM layout including shifters and assist shiftersin an exemplary configuration.

FIG. 3E illustrates a FPSM layout in which multiple shifters in a commonarea can be cut to resolve a phase conflict.

FIG. 4 illustrates an exemplary technique of making an FPSM.

FIG. 5 illustrates an exemplary technique for patterning a metal layerwith a FPSM.

DETAILED DESCRIPTION OF THE FIGURES Overview of Phase Shifting forNon-Damascene Layers

In accordance with one aspect of the invention, a type of phase shiftingmask (PSM) can be advantageously used in a damascene process withpositive photoresist. In a PSM, complementary phase shifters (alsocalled shifters) are configured such that the exposure radiationtransmitted by one shifter is approximately 180 degrees out of phasewith the exposure radiation transmitted by the other shifter. Therefore,rather than constructively interfering and merging into a single image,the projected images destructively interfere where their edges overlap,thereby creating a clear and very small low intensity image between thepair of shifters. This low intensity image generally represents afeature on the layout.

For example, in one embodiment, the shifters can be used to printcritical features of a layout. These critical features can beuser-defined and could include the gates of transistors. In a standardprocess, this PSM can be used in conjunction with a clear field trimmask that defines other features of the layout.

Overview of Phase Shifting for Damascene Layers

In accordance with one feature of the invention, instead of defining afeature by the low intensity area between the shifters, the highintensity areas created by the shifters can define the features. Thus,the inherent qualities of a PSM and a positive photoresist facilitatethe conversion of an original layout to a PSM layout that can be used ina damascene process. Specifically, an original layout can be easilyconverted to a PSM layout by replacing features with shifters.

In one embodiment, the phase shifters can be formed on a full phaseshifting mask (FPSM), which can define substantially all of the desiredfeatures of a layout for the metal layer. This FPSM can be used inconjunction with a dark field trim mask that can further define theareas of features left unexposed by the FPSM (explained below). Forexample, FIG. 1A illustrates a FPSM layout 100 that can be used in adamascene process for forming features in a metal layer. FPSM layout 100includes shifters 101, 102, 103, and 104, wherein shifters 101 and 103could provide 0 degree phase, whereas shifters 102 and 104 could provide180 degree phase. The desired three line pattern is shown best in FIG.1B by the dashed lines.

Note that the phase assignments discussed herein are illustrative only.Thus, shifters 101 and 103 could be 180 degree shifters, whereasshifters 102 and 104 could be 0 degree shifters. Moreover, shifters 101and 103 could be 185 degree shifters, and shifters 102 and 104 could be5 degree shifters. The important aspect is that adjacent shifters have aphase difference of approximately 180 degrees.

To conform to this requirement, a cut 105 can be provided, therebyresolving a potential phase conflict when assigning phase to theshifters of FPSM layout 100. Note that cut 105 results in an unexposedregion between phase shifter 102 and phase shifter 103. However, a trimlayout 110, shown in FIG. 1B, can expose this remainder of the feature,i.e. by exposing the photoresist in that area. Specifically, trim layout110 can include a cut 111 (which is substantially the size of cut 105)to account for the adjacency of shifters 101 and 102 as well as theadjacency of shifters 103 and 104. Note that trim layout 110, whichincludes the target layout (shown in dashed lines) for context, wouldactually include only cut 111 (shown in white). In one embodiment, therelationship between the width of the shifters and that of the printedlines can be 1-1. In other words, a 100 nm wide shifter can roughlydefine a 100 nm wide metal line. Note that proximity effects can affectthis width. Therefore, appropriate correction to the shifter can be usedto more closely approximate the desired line width.

FIG. 1C illustrates an aerial image 120 that could be formed by exposinga mask implementing FPSM layout 100 as well as a mask implementing trimlayout 110. In this case, the trim mask was exposed to twice the energyof the FPSM (referenced as a 1:2 exposure ratio). In other words, if theFPSM was exposed to N mJ/cm², then the trim mask was exposed to 2NmJ/cm². The exposure conditions for aerial image 120 were a wavelength(λ) of 193 nm, a partial coherence (σ) of 0.4, and a numerical aperture(NA) of 0.85.

The blue portion of aerial image 120 indicates a low intensity, the redportion indicates a high intensity, the yellow portion indicates anintermediate intensity, etc. The high intensity correlates to a highexposure, whereas the low intensity correlates to a low exposure. Asevidenced by the yellow and red bands in aerial image 120, thetransition from high to low intensity is abrupt, thereby resulting inwell-defined features. Specifically, aerial image 120 illustrates theformation of three lines 121, 122, and 123 (which correspond to thetarget layout shown as dashed lines in FIG. 1B). The predicted printededge is shown as a black line in the aerial image.

In accordance with one aspect of the invention, lines 121, 122, and 123could represent exposed areas of an oxide layer following development ofa positive photoresist layer. After etching these exposed areas, acopper layer could be deposited and planarized using the above-describeddamascene process, thereby forming three copper lines on the wafer.Advantageously, because the damascene process can include thedevelopment of a positive photoresist, the printing resolution of themetal pattern can be optimized.

Note that the cut and phase assignment described in reference to FIG. 1Acan be used on any region that includes a bend and tends to print large.In other words, the cut and phase assignment can be used on many cornersof a FPSM layout.

To further improve lithographic performance, various modifications canbe made to a layout to compensate for various proximity effects. Thesemodifications are called proximity corrections. One type of proximitycorrection, called optical proximity correction (OPC), appliessystematic changes to geometries of the layout to improve theprintability of a wafer pattern in response to a variety of proximityeffects, e.g. etch, resist, micro-loading, other proximity effects,and/or combinations of proximity effects.

Rule-based OPC can include rules to implement certain changes to thelayout, thereby compensating for some lithographic distortions thatoccur when printing the features onto the wafer. For example, tocompensate for line-end shortening, rule-based OPC can add a hammerheadto a line end. Additionally, to compensate for corner rounding,rule-based OPC can add (or subtract) serif shapes from outer (or inner)corners. These changes can form features on the wafer that are closer tothe original intended layout.

In model-based OPC, a real pattern transfer can be simulated (i.e.predicted) with a set of mathematical formulas (i.e. models). Inmodel-based OPC, the edges of a feature in a layout can be dissectedinto a plurality of segments, thereby allowing these segments to beindividually moved to correct for proximity effects. The placement ofthe dissection points is determined by the feature shape, size, and/orposition relative to other features.

FIG. 2A illustrates an FPSM layout 200, similar to FPSM layout 100 butincluding several proximity corrections, which can be used in adamascene process. FPSM layout 100 includes shifters 201, 202, 203, and204, wherein shifters 201 and 203 could provide 0 degree phase, whereasshifters 202 and 204 could provide 180 degree phase. Once again,shifters 201-204 are formed in a dark field mask.

To conform to a configuration in which adjacent shifters have a phasedifference of approximately 180 degrees, a cut 205 can be provided,thereby resolving a potential phase conflict when assigning phase to theshifters of FPSM layout 200. To expose the extraneous feature created bycut 205, a trim layout 210 can be provided, as shown in FIG. 2B. Trimlayout 210 includes a cut 211, which is substantially the size of cut205, to account for the adjacency of shifters 201 and 202, shifters 201and 204, as well as shifters 203 and 204. Note that cut 211 can includeproximity corrections including edge modifications as well as one ormore cuts therein.

In one embodiment, additional cuts on trim mask 210 can be used toprovide critical dimension (CD) control. U.S. Provisional PatentApplication 60/359,909, entitled “Non-Critical Blocking for Full PhaseMasks”, filed on Feb. 26, 2002 by Numerical Technologies, Inc., andincorporated by reference herein, describes such additional cuts.

FIG. 2C illustrates an aerial image 220 that could be formed by exposinga mask implementing FPSM layout 200 as well as a mask implementing trimlayout 210. Once again, the trim mask was exposed to twice the energy ofthe FPSM (referenced as a 1:2 exposure ratio). To permit comparisons,the exposure conditions for aerial image 220 are identical to those usedfor aerial image 120, e.g. wavelength (λ) of 193 nm, a partial coherence(a) of 0.4, and a numerical aperture (NA) of 0.85. Standard OPCparameters, which refer to the segment lengths after dissection, can beused for aerial image 220. In one embodiment, the OPC parameters caninclude 20 nm for the FPSM mask and 40 nm for the trim mask.

As evidenced by the substantially red bands in aerial image 220, thetransition from intermediate to low intensity is even more abrupt thanin aerial image 120, thereby resulting in three extremely well definedfeatures 221, 222, and 223. Thus, compared to the non-OPC result (i.e.aerial image 120), the resulting OPC image (i.e. aerial image 220)improves straightness of the edges of the three lines as well asintensity. In one embodiment, both aerial images 120 and 220 illustrate100 nm features. The predicted printed edge is shown as a black line inthe aerial image. FIGS. 1C and 2C are shown on the same page toillustrate the improvements provided by OPC.

In accordance with one embodiment of the invention, assist shifters(which because of their small size do not print, but nonetheless aid inprinting resolution) can be used to define isolated, and semi-isolatedmetal lines. In the case of densely packed metal lines, the phase ofeach metal line can be alternated to provide better feature definitionand the semi-isolated metal lines on the end can receive assist shiftersas well. Specifically, assist shifters could be used to improve theprinting of an isolated metal line. For example, FIG. 3A illustrates aFPSM layout 300 including an isolated metal line that will be definedusing a phase shifter 301. By adding assist shifters 302 and 303, placedon either side of and out of phase with shifter 301, the isolated metalline can be defined more easily.

FIG. 3B illustrates a FPSM layout 310 including multiple densely packedmetal lines that will be defined using shifters 312-316 (whereinadjacent shifters have opposite phase). In the case of semi-isolatedfeatures on the ends of the row, e.g. shifters 312 and 316 that have noprintable, proximate features, assist shifters can be used. Therefore,in this example, shifters 312 and 316 can have assist shifters 311 and317, respectively, placed alongside their isolated edges to improvetheir printing. The assist shifters are out of phase with the phase ofthe features themselves.

FIG. 3C illustrates a FPSM layout 320 including intermediate spaced orsemi-isolated (i.e. between isolated and densely packed) metal linesthat will be defined by shifters 322, 324, 326, and 328 withinterspersed assist shifters 321, 323, 325, 327, and 329. Once again,the phase of adjacent shifters/assist shifters have opposite phase. Notethat the configuration of the shifters including assist shifters (e.g.FIGS. 3A-3C) and determining phase assignment of those shifters can be afunction of pitch in the layout as well as an exposure setting.

FIG. 3D illustrates an FPSM layout 330 including shifters 331 and 334for printing metal lines in a U-shape configuration. As shown, all butone corner (cut 338) of the U will be defined on the phase shiftinglayer (with that corner defined by an opening, or cut, on the trimlayout (not shown)). To improve printing of the features correspondingto those shifters, assist shifters 335, 336, and 337 can be added toFPSM layout 330. In one embodiment, although shifters 331 and 334actually define the same feature, a cut 338 is placed to resolve apotential phase conflict between the U-shaped metal line and otherfeatures (not shown). In this case, an associated dark field trim mask(not shown) could include an appropriate cut to expose the areacorresponding to cut 338, c.f. FIG. 1B.

FIG. 3E illustrates a FPSM layout 350 including shifters 351, 352, 353,and 354, wherein all shifters correspond to individual features, e.g.two metal lines along side two shorter metal lines. Because of phaseassignments made in other part of the layout (not shown), phaseconflicts are created between shifters 351 and 352 as well as betweenshifters 353 and 354. In one embodiment, these shifters can be cut inthe areas designated by cut-lines 355 and 356. In that case, the upperportion of shifter 351 can be switched to a phase opposite that thelower portion. In a similar manner, the lower portion of shifter 354 canbe switched to a phase opposite that the upper portion. In this case, anassociated dark field trim mask could expose the areas where the cutswere made in shifters 351 and 354.

FIG. 4 illustrates an exemplary technique 400 of making a FPSM and trimmask. In step 401, a layout for defining a plurality of features in ametal layer can be received (e.g. the target or desired layout). Thislayout could be included in a GDS II file or other appropriate format.In step 402, the layout can be converted, if necessary, so thatsubstantially all of the features in the layout are defined by shiftersin a FPSM layout. In one embodiment, both critical and non-criticalfeatures can be represented by shifters in a one-to-one correspondence.This conversion can further include finding phase conflicts, cuttingshifters as appropriate to resolve these phase conflicts, placing assistfeatures as needed to improve printing resolution, and generating a trimmask layout based on the FPSM layout. The converted layout (includingboth the FPSM and trim layouts) can then be transferred to the physicalmasks (or reticle) using a known mask writing process in step 403. Themask set can be used for patterning a metal layer, such as copper, in adamascene process.

FIG. 5 illustrates an exemplary technique 500 for patterning that metallayer. In step 501, an oxide layer can be deposited on a wafer. In step502, a positive photoresist layer can be deposited on the oxide layer.At this point, the positive photoresist layer can be exposed with a FPSMand a trim mask in step 503. The FPSM includes a plurality of shifters,wherein the shifters represent most features in the metal layer. In oneembodiment, the trim mask is a dark field trim mask with at least onecut. This cut corresponds to a cut on the FPSM, wherein the cut on theFPSM resolved a phase conflict on the FPSM. In step 504, the positivephotoresist layer can be developed. At this point, the oxide layer canbe etched in step 505, thereby transferring the desired pattern to theoxide layer. In step 506, the metal layer can be deposited on the wafer.In step 507, the metal layer can be planarized to substantially a topsurface of the etched oxide layer. In this manner, the desired patternhas been transferred to the metal layer without etching of the metal.This damascene process is particularly useful for hard-to-etch metals,such as copper.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying figures, it is to beunderstood that the invention is not limited to those preciseembodiments. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed. As such, many modificationsand variations will be apparent.

For example, instead of cutting a feature at a corner (e.g. FIGS. 1A,2A, 3D, and 3E), the cut could be made in the line. Thus, referring toFIG. 1A, in lieu of cut 105, a cut could be made in shifter 104 atposition 106. The techniques described herein can be applied to masklayouts for various lithographic process technologies, includingultraviolet, deep ultraviolet (DUV), extreme ultraviolet (EUV), x-ray,etc. Accordingly, it is intended that the scope of the invention bedefined by the following Claims and their equivalents.

1. A method of generating a mask set, the mask set used for forming a Tfeature in a metal layer using a damascene process, the T featureincluding three lines, the method comprising: designing a phase shiftingmask (PSM) to form a first portion of the T feature in the metal layer,wherein designing the PSM includes: using a first shifter for formingpart of a top portion of the T feature; using a second shifter forforming another part of the top portion of the T feature and a part of abase portion of the T feature; using a third shifter for forming yetanother part of the top portion of the T feature; and using a fourthshifter for forming another part of the base portion of the T feature;and designing a dark field trim mask to form a second portion of the Tfeature in the metal layer, wherein the trim mask includes at least afirst cut, the first cut corresponding to a second cut between the thirdand fourth shifters on the PSM, the second cut resolving a phaseconflict on the PSM.
 2. The method of claim 1, wherein designing the PSMincludes providing proximity corrections for at least one of the first,second, third, and fourth shifters.
 3. The method of claim 1, whereindesigning the dark field trim mask includes providing proximitycorrections for the first cut.
 4. A layout for a dark field phaseshifting mask (PSM), the dark field PSM used to form a plurality offeatures on a metal layer, the metal layer forming one layer of anintegrated circuit, the layout comprising: a T feature including threenon-intersecting lines, the T feature comprising: a first shifter forforming part of a top portion of the T feature; a second shifter forforming another part of the top portion of the T feature and a part of abase portion of the T feature; a third shifter for forming yet anotherpart of the top portion of the T feature; and a fourth shifter forforming another part of the base portion of the T feature, wherein a cutbetween the third and fourth shifters resolves a phase conflict on thedark field PSM.
 5. The layout of claim 4, wherein at least one of thefirst, second, third, and fourth shifters includes proximitycorrections.
 6. A mask set used for forming a T feature in a metal layerusing a damascene process, the T feature including three lines, the maskset comprising: a phase shifting mask (PSM) to form a first portion ofthe T feature in the metal layer, the PSM including: a first shifter forforming part of a top portion of the T feature; a second shifter forforming another part of the top portion of the T feature and a part of abase portion of the T feature; a third shifter for forming yet anotherpart of the top portion of the T feature; and a fourth shifter forforming another part of the base portion of the T feature; and a darkfield trim mask to form a second portion of the T feature in the metallayer, wherein the dark field trim mask includes at least a first cut,the first cut corresponding to a second cut between the third and fourthshifters on the PSM, the second cut resolving a phase conflict on thePSM.
 7. The mask set of claim 6, wherein at least one of the first,second, third, and fourth shifters includes proximity corrections. 8.The mask set of claim 6, wherein the first cut includes proximitycorrections.
 9. A method of forming features in a metal layer using adark field mask, the method comprising: for an isolated feature,providing a shifter having a first phase, the shifter forming a feature;and positioning assist features on either side of the shifter, theassist features having a second phase, and the assist features beingsub-resolution features, and the assist features being sub-resolutionfeatures.
 10. A dark field mask for forming features in a metal layer,the dark field mask comprising: a shifter having a first phase, theshifter forming an isolated feature; and assist features on either sideof the shifter, the assist features having a second phase, and theassist features being sub-resolution features, and the assist featuresbeing sub-resolution features.
 11. A method of forming features in ametal layer using a dark field mask, the method comprising: for asemi-isolated feature bordering a densely packed feature area, providinga shifter having a first phase, the shifter forming a feature; andpositioning an assist feature on a first side of the shifter, the secondside of the shifter bordering the densely packed feature area, theassist feature having a second phase, and the assist feature being asub-resolution feature, and the assist features being sub-resolutionfeatures.
 12. A dark field mask for forming features in a metal layer,the dark field mask comprising: a shifter having a first phase, theshifter forming a semi-isolated feature bordering a densely packedfeature area; and an assist feature on a first side of the shifter, thesecond side of the shifter bordering the densely packed feature area,the assist feature having a second phase, and the assist feature being asub-resolution feature, and the assist features being sub-resolutionfeatures.
 13. A method of forming features in a metal layer using a darkfield mask, the method comprising: for an intermediate spaced featurearea, providing shifters having a first phase, each of the shiftersforming a feature; and positioning an assist feature between each pairof shifters, the assist feature having a second phase, and the assistfeature being a sub-resolution feature, and the assist features beingsub-resolution features.
 14. The method of claim 13, wherein if ashifter has an isolated side, then further including positioning anotherassist feature on the isolated side, the other assist feature having thesecond phase.
 15. A dark field mask for forming features in a metallayer, the dark field mask comprising: shifters having a first phase,each of the shifters forming a feature in an intermediate spaced featurearea; and an assist feature between each pair of shifters, the assistfeature having a second phase, and the assist feature being asub-resolution feature, and the assist features being sub-resolutionfeatures.
 16. The dark field mask of claim 15, wherein if a shifter hasan isolated side, then further including another assist feature on theisolated side, the other assist feature having the second phase.
 17. Amethod of forming a U shape feature in a metal layer using a dark fieldmask and a dark field trim mask, the method comprising: for the darkfield mask, providing a corner shifter having a first phase; providing afirst assist feature adjacent one outer edge of the corner shifter, thefirst assist feature having a second phase; providing a second assistfeature adjacent another outer edge of the corner shifter, the secondassist feature having the second phase, wherein the corner shifter, thefirst assist feature, and the second assist feature form first andsecond sides of the U shape feature; providing a line shifter having thesecond phase; providing a third assist feature adjacent an edge of theline shifter, the third assist feature having the first phase, whereinthe line shifter and the third assist feature form a third side of the Ushape feature; and providing a cut on the dark field trim mask, whereinthe cut corresponds to a space between the corner shifter and the lineshifter; and exposing the dark field mask and the dark field trim mask,wherein the each assist feature is a sub-resolution feature, wherein theassist features being sub-resolution features.
 18. A mask set forforming features in a metal layer using a dark field mask and a darkfield trim mask, the mask set comprising: for the dark field mask, acorner shifter having a first phase; a first assist feature adjacent oneouter edge of the corner shifter, the first assist feature having asecond phase; a second assist feature adjacent another outer edge of thecorner shifter, the second assist feature having the second phase,wherein the corner shifter, the first assist feature, and the secondassist feature form first and second sides of a U shape feature; a lineshifter having the second phase; and a third assist feature adjacent anedge of the line shifter, the third assist feature having the firstphase, wherein the line shifter and the third assist feature form athird side of the U shape feature, wherein each assist feature is asub-resolution feature, wherein each assist features is a sub-resolutionfeature.
 19. The mask set of claim 18, further comprising: for the darkfield trim mask, a cut corresponding to a space between the cornershifter and the line shifter.